1. Field of the invention
The present invention relates to impurity doping method utilized for forming a semiconductor region having a desired conductivity type and resistivity in fabrication process of a semiconductor device typically in the form of bipolar transistor or insulating gate field effect transistor.
The conventional ion implantation method is one of the typical impurity doping technologies. According to the principles of the ion implantation method, as shown in FIG. 23, firstly, an ion source 16 ionizes impurity atoms, then mass-separation is carried out by a magnet 17, and further an accelerating tube 18 is operated to effect accelerating of ions to a given energy level so as to implant accelerated ions into a surface of a sample, thereby effecting accurate control of impurity doping amount and facilitating impurity doping through an insulating film. For this reason, the ion implantation method has been generally utilized as the impurity doping technology in the conventional semiconductor fabrication process.
However, the conventional ion implantation method has various drawbacks due to its inherent nature as follows:
(i) Damage may be caused on a sample surface due to the kinetic energy of impurity ions to be implanted.
(ii) Since the implanted impurity atoms are distributed in the normal distribution having variance determined by their acceleration energy, it is impossible to form a steep density profile in a deep section as shown in FIG. 24.
(iii) With micronization of semiconductor device, asymmetricity of the device characteristics may be caused due to shadow effect.
(iv) It cannot be easy to form a shallow junction as channeling would be caused.
(v) If ion implantation is carried out with reduced acceleration voltage of impurity ion in order to form a shallow junction, converging degree of ion beam is degradated, resulting in reduction of production throughput.